In an effort to further understand the basis for the changes in steroidogenesis known to occur during sexual maturation in the rat, we examined by quantitative morphologic methods the number and ultrastructure of Leydig cells in fetal rats (days 18-20 of gestation) and in rats from days 2 to 3 of age through adult. Quantitative light microscopic analyses indicated that Leydig cell number, when expressed per unit volume of testis, was very high in fetal rat testes, fell significantly in testes of days 2 to 3 rats, and subsequently rose significantly. When Leydig cell number was expressed per testis rather than per unit volume of testis, the results indicated that testes of fetal rats and rats of days 2 to 3 contained the same number of Leydig cells; after the neonatal period, significant increases in Leydig cell number per testis occurred in concert with increases in testis weight. Quantitative electron microscopic studies revealed significant differences in the ultrastructure of fetal and adult populations of Leydig cells. For example, Leydig cells of fetal and neonatal rats contained abundant lipid, whereas Leydig cells of weeks 7 to 8 and adult rats contained little. Stereological analyses also revealed dramatic changes in smooth endoplasmic reticulum and inner mitochondrial membrane surface areas during sexual maturation, both per cell and per testis. These findings are discussed with respect to the steroidogenic capacity of the testis during sexual maturation.